Since laminated glass generates only a small amount of scattering glass fragments even when subjected to external impact and broken, it is excellent in safety. As such, the laminated glass is widely used for automobiles, railway vehicles, aircraft, ships, buildings and the like. The laminated glass is produced by sandwiching an interlayer film between a pair of glass plates.
In recent years, for the purpose, of attaining reduced weight of laminated glass, a technique of thinning the laminated glass has been studied. However, when the laminated glass is thinned, the sound insulating properties are lowered. In the case of using laminated glass low in sound insulating properties for a windshield of an automobile, or the like, there is a problem that sufficient sound insulating properties are not attained against the sound in a sound area of 5000 Hz or so such as wind noises and driving sounds of a wiper device.
Accordingly, a technique of enhancing the sound insulating properties of laminated glass by modifications of materials for the interlayer film has been studied.
As an example of an interlayer film for laminated glass, the following Patent Document 1 discloses a sound insulating layer containing 100 parts by weight of a polyvinyl acetal resin with an acetalization degree of 60 to 85% by mole, 0.001 to 1.0 part by weight of at least one kind of metal salt among an alkali metal salt and an alkaline earth metal salt, and more than 30 parts by weight of a plasticizer. This sound insulating layer can be used alone as a single-layered interlayer film or can be layered on other layer(s) to be used as a multi-layered interlayer film.